1. Field of the invention
The invention relates to electronic and electro-optical components based on crystal materials (for example III-V materials) for applications to transmitting data over optical fibers at high bit rates.
The invention relates more specifically to an integrated monolithic optical component including an electronic element and an electro-optical element, such as a transistor connected to a modulator to implement voltage control of the modulator.
The present invention seeks to improve the performance of the combination consisting of the electronic element and electro-optical element.
2. Description of the prior art
A conventional electro-optical modulator includes a stack of layers grown epitaxially on a III-V semiconductor material substrate, usually an InP substrate. The successive layers consist of a bottom confinement layer, an active layer, and a top confinement layer, these three layers forming the active waveguide of the component. The active layer is generally a layer of an undoped single quaternary compound, for example InGaAsP. The confinement layers consist of a III-V material, for example InP, each doped with a different type of carrier and one forming the anode and the other forming the cathode.
The modulator is generally controlled by a voltage source, for example a transistor, whose impedance can be as high as 50 Ω. The resulting cut-off frequency is then given by the equation:fc=1/(2π(RS+RL)Cm) where RS≈5 Ω, the series resistance of the modulator,                RL is the impedance of the control source, and        Cm is the capacity of the modulator.        
It follows that the performance of the modulator is not optimized and that the range of operating frequencies of an InP electro-optical modulator is one fifth to one tenth of that imposed by the intrinsic limitations of the material. This is particularly prejudicial in the case of high bit rate applications using modulators operating at bit rates from 40 Gb/s to 160 Gb/s.
Consideration has been given to minimizing or even eliminating the transport of electrical charges between the source and the modulator, in order to reduce the impedance of the control source. This has been attempted by moving two elements physically close together. However, this solution does not totally prevent interference between the source and the modulator. A stray resistance of only 1 Ωcan reduce the bandwidth below 40 GHz.
Another prior art solution uses monolithic integration of the modulator with the control transistor by integrating the active layer of the modulator into the structure of the transistor (into the collector layer).
This solution has drawbacks, however. The active layer of the modulator is in the depletion area of the collector, and in the equivalent circuit diagram this amounts to connecting the modulator in series with the collector. As there is then no longer access to one layer of the modulator, current control is used rather than voltage control. Current control implies a high load resistance, which can even exceed 50 Ω. The bandwidth of the modulator is therefore not increased.
The object of the present invention is to propose integration of the opto-electronic component with the electrical control element in such a manner as to minimize stray resistances and enable optimum operation of the electro-optical component.
To this end, the invention proposes using a heterojunction bipolar transistor (HBT) as the voltage control source and monolithic integration of the electro-optical component into the HBT.
In accordance with the present invention, the electro-optical component is integrated into the sub-collector layer of the HBT, which advantageously constitutes one confinement layer of said component.
To this end, the active structure of the electro-optical component must be widened so that the sub-collector layer directly constitutes the top confinement layer of said component. The electro-optical component must be widened because the size of the transistor cannot be reduced below a limiting value imposed by fabrication and design constraints.
However, widening the active waveguide of the electro-optical component eliminates monomode propagation of the signal in the component. The invention consequently proposes to lengthen the widened area of the active waveguide of the component, which is situated under the HBT, in order to transform the widened area into a multimode interference coupler including a monomode input waveguide and a monomode output waveguide.